GIANT HYDRAULIC DUNES IN CARBONATES?
The studied escarpment is more than 150 m high. Three packages can be individualized. The lower package is around 15 meters thick and is made of shaly limestones. The middle part is up to 80 m thick and made of limestones to shaly limestones. Many dissolution-derived caves are located along this lower contact. The upper package is 30 meters thick and is made of limestones (calcaires miroitants). The middle package becomes thinner on the left of this picture to finally disappear between the lower and the upper package.
Many decametric to hectometric – scale clinoforms can be recognized into the middle package, using an accurate picture analysis. They are around 200 meters long and 80 meters high (picture 2).
This paleo sedimentologic feature is interpreted as a submarine hydraulic dune deposited in shallow waters (> 100 m) during the lower Cretaceous (Gotis, 1957). The progradation sense can be interpreted from clinoforms drawing (eastwards).
Was this hydraulic dune deposited into a sediment transfer zone leading to the deep Vocontien basin? Was it related to contour currents?
The studied sediments were deposited during the lower Cretaceous (Valanginian). The rocks are made of carbonates, mainly mudstones to wackstones, their matrix porosities are moderate in a reservoir point of view (around 10 %), far away from the dissolution caves.
The 3 carbonate packages are displaced by an apparent normal fault exhibiting an offset that is around 10 meters. This fault was active after lithification, one hundred millions years (100 Ma) following the sediments deposition.
HOW COULD WE USE THIS PICTURE?
The clinoforms scale should be easily identified using conventional seismic methods. However, the lithological contrast between clinoforms would be not enoughly developped to be properly imaged (a probable lack of contrast impedency). As a consequence, the sense of dune propagation into this package would be complex to determine using seismic analysis. In addition, the relative facies homogeneity seems tricky for correctly contextualizing such geologic feature using core data. Core data would not be ideal to catch such feature scales. Basically, we (the geoscientists) can be blind looking at such features using conventional methods. Only reservoir unit thickening would be imaged, therefore hiding geologic processes leading to this thickening.
Subseimic-scale structural heterogeneities:
The fault-zone would be recognized as a seismic lineament on a 3D slice assuming the 10 m – wide normal displacement. The fault zone can be targeted assuming the structural porosity that can be found into such damage zone.
This studied case could be used for reservoirs that are located into giant hydraulic carbonates dunes such as suggested by Fribourg (2010) for carbonates coastal dunes.
Feel free to constructively comment this open discussion. Feel free to provide relevant references where similar seismic-scale geometries have been recognized.
Some relevant scientific references:
Cornet, C., 1972, Etude géomorphologique du massif de l’Hortus (Hérault) : Etudes quaternaires, 1972, 1, 23-38.
Frebourg, G., 2010, Carbonate coastal dunes: potential reservoir rocks?. Thèse de doctorat : Univ. Genève, 2010, no. Sc. 4221
Gerbier, P., 2020, Petits secrets géologiques du Causse de l’Hortus et de ses environs. Bulletin N°99, Association Géologique d’Alès et de sa région, 64 p.
Gotis, M., 1957, Contribution à la connaissance géologique du Bas-Languedoc, Thèse d’Etat, Montpellier.
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